packer: always reject plugin pre-releases

When a pre-release version of a plugin is locally installed, it may or
may not be loaded depending on the constraints expressed in the
template being executed.

If the template contains constraints for loading the plugin, it would
be ignored, while if that wasn't present, it would be loaded.

This is inconsistent, and deserves to be addressed, which is what
this commit does.

With this change, plugin pre-releases are now always rejected with a
message in the verbose logs, so only releases are considered, whether or
not the template being processed contains a `required_plugins`
constraint or not.

packer/plugin-getter/plugins.go

@@ -159,6 +159,11 @@ func (pr Requirement) ListInstallations(opts ListInstallationsOptions) (InstallL
 			continue
 		}
 
+		if pv.Prerelease() != "" {
+			log.Printf("pre-release version of plugin %q discovered: unsupported, ignoring", path)
+			continue
+		}
+
 		if strings.Replace(pluginVersionStr, "v", "", -1) != describeInfo.Version {
 			log.Printf("plugin %q reported version %s while its name implies version %s, ignoring", path, describeInfo.Version, pluginVersionStr)
 			continue